The basic function of transport networks is transparent transport of client signals. A client signal may be transported within a transport network using transport containers. If the size (i.e., bit rate) of a client signal does not match the size of transport containers of the transport network, direct mapping of one client signal into one transport container in not a bandwidth-efficient solution. A typical solution is inverse multiplexing. At the ingress point of the transport network, a client signal is divided into multiple sub-signals where each sub-signal has a size close to the size of the transport container and is mapped into one transport container. At the egress point of the transport network, the sub-signals are extracted from the transport containers and the client signal is reconstructed from the sub-signals. The bandwidth efficiency is achieved by using sub-signals as close as possible in size to the transport containers, thereby minimizing unused bandwidth in the transport network.
For common transport networks (i.e., Synchronous Optical Networks (SONET), Synchronous Digital Hierarchy (SDH) networks, Optical Transport Networks (OTNs), and the like), an inverse multiplexing scheme known as virtual concatenation (VCAT) is used to transport client signals across the transport network in a bandwidth-efficient manner. The VCAT scheme is standardized by the International Telecommunications Union-Telecommunications (ITU-T). In VCAT, for each client signal to be transported across the transport network, the number of transport containers to be used to transport the client signal across the transport network is specified, and the client signal is divided into sub-signals by byte-stripping across the transport containers available for that client signal. In VCAT, the set of transport containers used to transport a client signal is a virtual concatenation group (VCG) having multiple VCG members (i.e., transport containers).
With respect to transmission quality, since a VCG signal has one-to-one correspondence with a client signal transported therein, transmission quality of the client signal may be assessed using transmission quality of the VCG signal. The existing VCAT standards imply monitoring individual VCG member signals of a VCG signal in order to assess the transmission quality provided to the client signal transported by that VCG signal; however, this requires establishing and managing a performance monitoring point for each VCG member signal in a VCG signal, for each VCG signal in the transport network. Disadvantageously, monitoring individual VCG member signals of VCG signals is costly in terms of both network resources (including network element resources and network management resources) and operational expenses (since each performance monitoring point must be managed individually).
Furthermore, in existing transport network equipment, each port unit typically includes anywhere from two to sixty-four VCGs, where each VCG includes between sixty-three and two hundred and fifty-six VCG members. Since existing transport network equipment typically includes between eight and thirty-two such port units, existing transport network equipment typically supports in excess of two thousand VCGs and several tens to hundreds of thousands of associated VCG members. Moreover, as technology advances between equipment generations, the numbers of port units, VCGs, and VCG members supported by transport network equipment continues to grow. This rapid growth of VCG capacity further exacerbates problems associated with transmission quality monitoring.